A SGI(Shallow Groove Isolation) structure is a structure which electrically insulates adjoining transistor on a semiconductor substrate. The SGI structure is formed following processes. Forming a groove at a silicon substrate, forming a pad oxide film on the silicon substrate, forming an oxidation inhibition film on the pad oxide film and afterwards, thermally oxidize the surface of the groove to form an element isolation thermal oxide film on the surface of the groove and burying in the groove an insulating film. Further formed thereon are several films including a gate oxide film, a gate electrode film, a dielectric film, a lead, an interlayer dielectric film, and so on to thereby obtain a semiconductor device.
This SGI structure becomes the suitable structure in the device after the 0.25 .mu.m process, because dimensional accuracy of the SGI structure is higher than conventional LOCOS(Local Oxidation of Si) structure of which has been used more than 0.25 .mu.m process. However, when thermally oxidize the surface of the groove to form the element isolation thermal oxide film, there is a case that the groove upper edge becomes acute angle. Such acute angled part remain of the groove upper edge, as A.Bryant et al disclosed in "Technical Digest of IEDM'94,pp. 671-674", an electric field concentration arises at the acute angled part under circuit operation, and, there is a case that decline in characteristics of transistors and voltage characteristics of capacitors.
As a solution of these problems, JP-A-2-260660 disclose that before thermally oxidize the surface of the groove to form an element isolation thermal oxide film, remove the pad oxide film about 0.1 .mu.m from sidewall in the groove upper edge. And after that, by oxidizing at the temperature about 1000.degree. C. with a steam, to roundly form the the groove upper edge.